The present invention relates to a numerical control system and in particular to a system that combines the best attributes of both analog and digital numerical control techniques.
The use of numerical control systems and in particular computerized numeric control to control the movement of machining operations has within recent years become quite widespread. Numerical control systems fall into two general categories: open loop systems and closed loop systems. In open loop systems, the real (actual) position of the head assembly being controlled must be determined by counting the number of counts (increments) above or below a zero position. Hence, the wear of all drive line components becomes a factor in positioning accuracy, requiring elaborate support systems to "compensate" for mechanical degradation. However, open loop systems have usually enjoyed a cost advantage over closed loop system since the added expense of such support systems is generally more than offset by the additional hardware, in the form of absolute encoders and precision date racks, required to close the loop.
A second dilemma facing the numerical control system designer has been whether to employ analog or digital circuitry for prime processing. On the one hand, basic analog controls are relatively straight forward, however the required degree of accuracy can only be obtained with costly, highly sensitive components and complex compensating circuitry to overcome the inherent drift problems in analog signals caused by changes in temperature, supply voltage, etc. With digital circuitry, on the other hand, accuracy is readily attainable, however, a massive array of digital logic circuits is required in order to accommodate large scale control.
Accordingly, it is the primary purpose of the present invention to provide an extremely accurate and cost effective closed loop numerical control system that combines the best attributes of both analog and digital control. Specifically, the present hybrid system gathers positional information in absolute form via an encoder and compares the information to the desired position digitally. The resulting digital signal is then processed in two distinct operations. For macro differences (large scale), the most significant bits of the digital signal are converted to an analog command by a "state-of-the-art" digital-to-analog converter chip. The D-to-A converter used may have relatively poor temperature and accuracy specifications in exchange for a very low cost. The poor specifications, however, are irrelevant since the functional responsibility of the device is macro (large scale) processing. For micro (small scale) differences, the least significant bits in the digital signal are processed by a novel network of polarity switched analog gates. In particular, each analog gate is fed with a polarity signal that determines the desired direction of movement. The control terminal of each gate is controlled by one of the least significant bit lines from the digital signal, and the output of each gate is tied to a current limiting (or voltage proprotioning) resistance. The various resistance lines are then combined to form a single (micro) analog command. The net result of the present invention is that expensive analog components are avoided and the amount of digital circuitry is minimized.
In addition, the preferred embodiment of the present invention herein includes a novel tach responsive braking generator circuit to accomplish the positioning of high mass heads without "hunting". Basically, the circuit comprises a double integrating tachometer that is responsive to the rate of change of the least significant bit in the output signal from the encoder to produce a signal whose magnitude is directly related to the velocity of the head assembly. The velocity signal is then provided through a level shifter to a bipolar output stage whose output polarity is opposite that of the motive drive signal. In operation, if during deceleration of the head assembly the encoder indicates that the velocity is not falling off at an appropriate rate, the tach responsive braking generator circuit is adapted to produce a reverse polarity braking signal that when combined with the motive drive signal serves to dynamically brake the head assembly.
Additional objects and advantages of the present invention will become apparent from a reading of the detailed description of the preferred embodiment which makes reference to the following set of drawings in which :